1. Field of the Invention
The present invention relates to the selection of an optimal swapping technique for a Discrete Multitone (DMT) system.
2. Brief Description of the Prior Art
Today's telephone networks (twisted pair copper media) are not originally designed for high data rate transmission. In order to accommodate high data rate interactive services provided for small businesses and general households, high-speed communication paths must be available. Though the optic fiber is the ultimate choice, the optic fiber network is not yet well-constructed at this stage. To solve this problem, Asymmetric Digital Subscriber Line (ADSL) technology has been proposed to increase the effective bandwidth of the existing telephone networks for allowing high-speed internet connection.
DMT systems are implemented in ADSL communication system for dividing the available bandwidth provided by twisted pair copper media into multiple sub-channels (i.e. tones). It has been adopted in many industrial standards, such as the ANSI T1.413. In order to maintain optimal performance in a DMT system, it is generally desirable that the mean square errors (MSE) should be kept as close as possible among different sub-channels.
Prior art bit-swapping techniques are used to reduce the difference between the maximum mean square error (MSEmax) and the minimum mean square error (MSEmin) within all sub-channels. This technique adjusts the sub-channels corresponding to MSEmax and MSEmin, while leaving the sub-channels with relatively moderate MSE unchanged. Details regarding the bit-swapping may be found in “Understanding Digital Subscribe Line Technology”, Prentice Hall PTR, Upper Saddle River, N.J. 07458, by Thomas Starr, John M. Cioffi, and Peter Silverman, and U.S. Pat. No. 5,479,447, “Method And Apparatus for Adaptive, Variale Bandwidth, High-Speed Data Transmission of a Multicarrier Signal Over Digital Subscriber Lines” granted to Chow et al. The detailed manner for calculating MSE respecting each sub-channel is well-known and may be found in any related reference.
Bit-swapping reallocates bits of data from the sub-channel with MSEmax to the sub-channel with MSEmin. In this way, the SNR (signal-to-noise ratio) of the sub-channel with MSEmax increases due to fewer bits of data loaded under the same gain. On the other hand, the SNR of the sub-channels with MSEmin decreases due to more bits of data loaded under the same gain. Since the increase of SNR implies the decrease of MSE, and vice versa, one reduces the difference between the MSEmax and MSEmin by reallocating amount of bits in each sub-channel. According to Chow's method, bit-swapping occurs when MSEmax>MSEmin+MSEthreshold, where MSEthreshold is typically chosen to be 3 dB.
Considering the example as shown in FIG. 1, MSEmax is 6.2 dB and MSEmin is 3 dB, and the gain factors corresponding to the two sub-channels are gmax=−1 dB and gmin=1.5 dB respectively. The difference between MSEmax and MSEmin is 3.2 dB. Since the difference between MSEmax and MSEmin exceeds 3 dB, bit-swapping is employed and the process ends up with MSEmax decreasing by 3 dB and MSEmin increasing by 3 dB. The resultant error level diagram is shown in FIG. 2. The updated difference between MSEmax and MSEmin is 2.8 dB. There is an improvement of 0.4 dB over the previous condition.
Bit-swapping suffers greatly from numerous drawbacks. It is not efficient enough for eliminating the difference between MSEmax and MSEmin. It is even useless in some specific situations. Considering the following example shown in FIG. 3, in which the difference between MSEmax and MSEmin is 3 dB. When bit-swapping technique is employed, MSEmax decreases 3 dB while MSEmin increases 3 dB. The resultant difference between MSEmax and MSEmin after bit-swapping remains 3 dB. Further bit-swapping will ultimately fail to reduce the difference between MSEmax and MSEmin. In the example of FIG. 3, it shows that performing bit-swapping in such situation is a waste of system resource.
The concept of gain-swapping has been set forth as an alternative method to reduce or even eliminate the difference among all the sub-channels. The gain-swapping concept features that the MSEs in all sub-channels can be brought to the same level by adding gain or subtracting gain from each channel. However, so far there has been no document proposed to discuss gain-swapping algorithm. Most publications focus on bit-swapping, and no document has ever shown that gain-swapping can provide better performance than bit-swapping does in some cases.